We try to analyse quantitatively the processes that intervene between the incidence of light on the retina and the transmission of the neural message from eye to brain: thus we are concerned with the optical properties of the retina, with the photosensitivities, optical densities and spectral sensitivities of the visual pigments, and with the size of the electrical signals generated by the receptors and the changes that they undergo in passing through the retina. Particular attention is paid to the problems of color vision and of light and dark adaptation. In trying to understand these processes we rely chiefly on subjective visual experiments, supplementing these by the methods of retinal densitometry and electroretinography.